Gonad Development of the Ark Shell Scapharca broughtonii (Schrenck) Broodstock in Farming Grounds of Japan

Abstract

Histological changes in gonad development of the ark shell Scapharca broughtonii were observed in 1-, 2-, and 3-year old broodstock from May to October 1984 and in 4-year old broodstock from May to December 1985. Animals were cultured in cages at the farming ground in Kasado Bay, Yamaguchi Prefecture. The spawning season of the ark shell broodstock cultured in Kasado Bay throughout the year was estimated to be from early July to middle September, and the maturative degrees of these ark shells were not sufficient. On the other hand, the spawning season of the ark shell broodstock which were cultured for 3 years in Kagawa Prefecture and transplanted to Kasado Bay in May 1985 was estimated to be from late May to middle October, and the maturative degrees of these shells were sufficient. It is thought that the nutritive and growth condition of the ark shell caused the differences of maturative degree. The sex ratios of 3-year old ark shells cultured in Kagawa Prefecture were 50% males, 44% females and 6% immature. In late June 1986, maximum fecundity of 3-year old ark shells cultured in Kagawa Prefecture was estimated to be about 1.2×107 per female. During histological observation of the ark shell gonads, only one harmaphrodite was found among the 741 samples prepared in this study.